UTI is an exceedingly common outpatient problem among young healthy women, resulting in considerable morbidity and health care costs. In addition, increasing antibiotic resistance, even among community-acquired UTIs, is making treatment of these infections more problematic. Thus, novel non-antimicrobial prevention strategies are urgently needed. One such approach for which there is strong evidence at the mechanistic level is use of an H2O2-producing lactobacillus probiotic to restore normal vaginal flora. If effective in reducing E. coli colonization and UTI, this strategy would reduce the costs and morbidity associated with urinary tract infections, including time lost from work, health care costs, antimicrobial use, and the ultimate development of antimicrobial resistance. Large, well controlled studies using a carefully selected probiotic strain are required to ascertain whether this approach is of preventive value. We thus propose a randomized, double-blind, placebo controlled Phase II proof of concept trial which will assess whether a specifically selected strain of Lactobacillus crispatus (the most prevalent lactobacillus species in the vagina) given intravaginally can restore the normal vaginal flora, reduce vaginal colonlization with uropathogens, and decrease the subsequent risk of UTI in women at high risk of recurrence. We will utilize L. crispatus CTV-05, a strain that is known to be highly adherent to vaginal epithelial cells and to produce H2O2 in high quantities. Premenopausal women who have a history of recurrent UTI will be randomized to treatment with a vaginal suppository containing L. crispatus or placebo. Vaginal cultures for lactobacilli and uropathogens will be done at baseline and on a monthly basis for four months. Lactobacilli will be characterized as to H2O2-production and repetitive sequence PCR will be used to identify the persistence of the probiotic strain. Urine cultures will be collected at baseline and monthly for four months and whenever UTI symptoms occur. We will assess the following outcomes of interest: 1) time to recurrent UTI; 2) incidence of asympotmatic bacteriuria; 3) incidence of E. coli vaginal colonization; and 4) treatment outcomes when the probiotic is used adjunctively for treatment of acute UTI. In addition, we will assess the rate and duration of vaginal colonization with two different dosing regimens (3 days versus 5 days) of the L. crispatus probiotic and correlate colonization with in vitro adherence to vaginal epithelial cells collected from the probiotic-treated women. This trial will assess the efficacy of the lactobacillus probiotic, L. crispatus CTV-05, in preventing E. coli vaginal colonization, asymptomatic bacteriuria, and UTI in women with recurrent UTI, and will also provide insight into the host and microbial mechanisms involved.